The present invention relates to a method and apparatus for linear and nonlinear control of a hydraulic press, and more particularly to a means for controlling a thermoset process.
Hydraulically operated thermoset presses are widely used in many materials-molding applications. Although different types of molding processes, such as compression, transfer, or injection molding, may be employed depending upon the nature of the molded material and the type of product to be formed, each process generally contemplates filling a cavity with a material, applying pressure to the material, and maintaining the pressure for a "set" or "cure" period.
Pressure is generally applied by a press consisting of a hydraulically operated piston movable in a cylinder containing the material to be molded. It is normally desirable to control the velocity of the material flowing through a mold cavity followed by control of the material pressure during the final stages of the filling of the cavity. Due to the physical characteristics of various types of molding materials as well as the complicated shapes of parts to be molded, nonlinear movement of the piston is required in order to achieve best results. Hydraulic presses currently in the art control either the pressure in front of the piston head or the velocity of the piston in order to control piston movement. Generally such control is accomplished be entering a "profile" of the desired piston movement with respect to distance or time into an electronic control device.
Such presses and associated control devices currently in use have the disadvantage of not being able to achieve zero or low velocities if the resistance of the material to movement is lower than the force developed by the minimum operating pressure of the system, which is normally in the range of 100-150 p.s.i. This pressure is developed from the resistance to the flow of hydraulic fluid from the pump to a tank as a result of pressure drops occurring in the lines and valves. Because the hold portion of the cycle generally constitutes the major portion of a cycle, a large amount of energy is wasted, due to the use of a fixed volume pump.
In accordance with the principles of the present invention a method and apparatus for linear and nonlinear control of hydraulic presses utilizes a sequence valve which is set just above a minimum operating pressure of the system, to prevent pressure and flow from reaching the piston to be controlled. When movement of or pressure to the controlled piston is desired, the pressure will exceed the sequence valve setting, thereby opening the valve and allowing hydraulic supply to reach the piston. The method and apparatus also includes a directional valve movable to operate a pump as a fixed displacement pump during a controlled portion of a molding cycle, and movable to allow a proportional pressure controller to control the pump compensator during a hold portion of the cycle, so that the pump may operate at substantially reduced horse power during the major portion of the cycle.
The inventive concept is usable to move a piston in a cylinder to fill a cavity, and is also usable to operate a variable volume vane pump.
During a zero pressure or velocity portion of the cycle of operation, a spring biased spool is moved by hydraulic pressure to connect the pump output to a reservoir, and a spring biased sequence valve blocks application of hydraulic pressure to the press.
During a controlled-movement portion of the cycle, the spool is moved to allow a selected proportion of the pump output to reach the press, and such proportion is controlled by a feedback signal to provide pressure or velocity compensation as required.
During a hold or cure portion of the cycle, a directional valve is moved to allow a relief valve contained in the pump to operate so that the pump is changed from a fixed output pump to a variable output pump and can be operated at substantially reduced horsepower resulting in substantial energy savings. It is an object of the present invention to provide a method of controlling the speed of a piston at low velocities down to zero velocity without external resistance to movement.
It is a further object of the invention to utilize a hydraulic pump operable in fixed and variable output modes and a means for switching from one mode of operation to another, so that the pump may be operated in the variable output mode whenever possible to conserve energy.
Other objects and advantages of the invention will be apparent from the following detailed description and associated drawings.